If you need to do it within FreeSWITCH (eg from a script), execute the uuid_set_media_stats api. This will set a number of variables that you can then read which'll reflect a number of RTP statistics. This will include the number of RTP frames/packets/bytes going in and out of the server for that call. That'll let you spot if a channel isn't receiving any RTP packets.

To see what variables are set see the add_stat lines in src/switch_core_media.c:1470 set_stats(), the names will be prefixed between what appears in the code and the variable set for example "in_raw_bytes" => ${rtp_audio_in_raw_bytes}.